Mollusks to Hungry Lobsters: Snot Gonna Happen!

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Spiny lobsters have hard shells and strong jaws easily capable of
turning the soft-bodied, sluglike sea hare into an easy meal. But
new research finds that sea hares have a snotty solution: Clog up
potential lobster predators' nostrils.

When threatened, the sea
hares excrete a white, sticky substance called opaline that
stuffs up the lobsters' sensory organs. While the lobster
struggles to deal with this sudden lack of smell, the sea hare
can often escape its clutches.

Sea hares (Aplysia) are marine mollusks that get their
name from two long, earlike projections on their heads. The
largest species can grow up to 4.4 pounds (2 kilograms).

Chemical protection

Size offers some protection from potential predators, but sea
hares' main defenses are chemical. Like squid, they release
purplish ink when threatened, but also excrete opaline. [ See
Video of Snotty Sea Hare in Action ]

Georgia State University researchers knew that these chemicals
could save sea hares from becoming a meal to spiny
lobsters, but it wasn't clear whether the substances simply
blocked the lobster sensory organs (which are on its antennae) or
whether they actually induced chemical signals that prevented the
crustaceans from smelling food.

To find out, the researchers used Caribbean spiny lobsters
(Panulirus argus) and sea hares (Aplysia
californica). They extracted the water-soluble part of the
sea hares' opaline, which left them with all of the stickiness
but none of the
amino acids and other chemicals that might have affected the
lobsters' chemosensory receptors. They painted the opaline
abstract on the lobsters' antennae and then exposed the
crustaceans to "shrimp juice," which was made by soaking shredded
shrimp in water for an hour.

The researchers repeated the same experiment with three other
substances: carboxymethylcellulose, which is sticky but lacks
opaline's amino acids; a mixture of only the amino acids found in
opaline, without the stickiness; and a combination of
carboxymethylcellulose and the opaline amino acids. A final group
of lobsters got to smell the shrimp juice with nothing blocking
their antennae.

As the lobsters responded to the shrimp juice in each condition,
the researchers measured the activity of their chemosensory
neurons.

Sticky defense

The results revealed that even without neuron-affecting
chemicals, opaline's stickiness alone is enough to save sea hares
from spiny lobsters. The amino acid-free carboxymethylcellulose
had the same effect as opaline, the researchers report today
(March 27) in the Journal
of Experimental Biology. The opaline amino acids alone,
however, did not stop the lobster neurons from responding to
smells, perhaps in part because they were easily washed away by
sea water.

Spiny lobsters have chemical sensory organs all over their head
and legs, and the researchers suspect sea hares can clog them
all.

"Typically, a sea hare is in the grasp of a spiny lobster before
the sea hare inks," they wrote. "Our observations are that the
ink sticks to all of the sensory appendages in the anterior end,
including the antennules, mouthparts and anterior legs. We would
expect an effect on these other chemoreceptors similar to that we
have demonstrated for antennular chemoreceptors."